Re: [PATCH 2/6] x86/sev-es: Forward page-faults which happen during emulation
From: Sean Christopherson <seanjc@google.com>
Date: 2021-05-12 19:36:48
Also in:
kvm, lkml, stable
On Wed, May 12, 2021, Joerg Roedel wrote:
quoted hunk ↗ jump to hunk
From: Joerg Roedel <redacted> When emulating guest instructions for MMIO or IOIO accesses the #VC handler might get a page-fault and will not be able to complete. Forward the page-fault in this case to the correct handler instead of killing the machine. Fixes: 0786138c78e7 ("x86/sev-es: Add a Runtime #VC Exception Handler") Cc: stable@vger.kernel.org # v5.10+ Signed-off-by: Joerg Roedel <redacted> --- arch/x86/kernel/sev.c | 4 ++++ 1 file changed, 4 insertions(+)diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c index c49270c7669e..6530a844eb61 100644 --- a/arch/x86/kernel/sev.c +++ b/arch/x86/kernel/sev.c@@ -1265,6 +1265,10 @@ static __always_inline void vc_forward_exception(struct es_em_ctxt *ctxt) case X86_TRAP_UD: exc_invalid_op(ctxt->regs); break; + case X86_TRAP_PF: + write_cr2(ctxt->fi.cr2); + exc_page_fault(ctxt->regs, error_code); + break;
This got me looking at the flows that "inject" #PF, and I'm pretty sure there
are bugs in __vc_decode_user_insn() + insn_get_effective_ip().
Problem #1: __vc_decode_user_insn() assumes a #PF if insn_fetch_from_user_inatomic()
fails, but the majority of failure cases in insn_get_seg_base() are #GPs, not #PF.
res = insn_fetch_from_user_inatomic(ctxt->regs, buffer);
if (!res) {
ctxt->fi.vector = X86_TRAP_PF;
ctxt->fi.error_code = X86_PF_INSTR | X86_PF_USER;
ctxt->fi.cr2 = ctxt->regs->ip;
return ES_EXCEPTION;
}
Problem #2: Using '0' as an error code means a legitimate effective IP of '0'
will be misinterpreted as a failure. Practically speaking, I highly doubt anyone
will ever actually run code at address 0, but it's technically possible. The
most robust approach would be to pass a pointer to @ip and return an actual error
code. Using a non-canonical magic value might also work, but that could run afoul
of future shenanigans like LAM.
ip = insn_get_effective_ip(regs);
if (!ip)
return 0;